Gas Phase Protolysis of Trisarylzincate Anions

Abstract

We have applied a combination of tandem-mass spectrometry, quantum-chemical calculations, and statistical rate theory computations to examine the gas phase reactions between the trisarylzincate anions ArXZnPh2 – (ArX = p-X-C6H4, X = NMe2, OMe, Me, H, F, and Cl) and 2,2,2-trifluoroethanol at T = 310 ± 20 K. The observed reactions bring about the protonation of one of the aryl anions, which is then released as the corresponding arene, while the formed alkoxide binds to the zinc center. The protonation is faster for the more electron-rich aryl groups and shows a linear Hammett plot if the rate constant for X = NMe2 is discarded from the analysis. Although the reactions are highly exothermic, they proceed only with relatively low efficiencies (0.1% ≤ φ ≤ 1.3%). According to the quantum-chemical calculations, this behavior can be ascribed to the reactions proceeding through a double-well potential with a tight transition structure located at the central barrier. Based on these potential energy surfaces, the statistical rate theory computations can reproduce the measured rate constants within factors of 2 to 8. A comparison of the protolysis of the trisarylzincates with that of the corresponding free aryl anions demonstrates how the coordination to the metal center not only stabilizes the carbanions energetically but also moderates their reactivity. Thus, our gas phase study contributes to a better understanding of the fundamentals of organometallic reactivity.

Anionic Dimers of Fluorinated Alcohols

“…From the equilibrium constants K , the Gibbs reactions energies Δ G ° were then calculated according to eq , with R being the gas constant and T the temperature. In line with previous work on three-dimensional quadrupole ion traps, we assume a temperature of T = 310 ± 20 K. ,, …”

Electrospray-Ionization Mass Spectrometry for the Analysis and Quantification of Carbanions

“…Theoretical alkali metal cation affinities for the 6 carbanions were also determined using the same method as for the calculation of the affinities of the benzoate anions that have been used as reference species (DLPNO-CCSD(T)/cc-pVQZ (M: cc-pwCVQZ)//PBE0 D3BJ/def2-TZVP). 215 The agreement between experimental and theoretical values was, with few exceptions, very good (Figure 37). The mean deviation between theoretical and experimental affinities to K + was 3 kJ/mol, while even the mean deviation for Li + was only 5 kJ/mol.…”

“…For this reason, calculated affinity values for a series of reference compounds were used. 215 The chosen compounds were a series of para-substituted benzoate anions pB R − (Table 7). They had recently been shown to be amenable to calculation via quantum chemical methods by confirming their relative affinities experimentally.…”

“…Systematic deviations of the absolute values cannot be precluded entirely, though two separate calculations using different methods deviated by less than 1 kcal/mol. 61,215 Furthermore, the interpretation of the mass spectrometric data using the kinetic method can easily be updated with more accurate reference values once they are available, either from improved calculations or from novel experimental methods. The ratio of benzoic acid to carbon acid was chosen due to a generally higher responsiveness of benzoate anions towards ESI-MS. A total of six carbon acids were selected from methylenes featuring a mix of two cyano, keto, ester or amide groups.…”

“…). 215 Every series of different fragmentation energies was considered separately in order to estimate the entropic contributions. The correlation of the logarithmic fragment ion ratios to the calculated alkali metal cation affinities of the benzoates was for the most part linear, as was expected.…”